In recent years, along with a decrease in power supply voltage and an increase in capacity of information devices, the voltage and current of a switching power supply tend to decrease and increase, respectively.
FIG. 1 is a view showing the typical arrangement of a transformer used in the switching power supply, and is disclosed in, e.g., Japanese Patent Laid-Open No. 6-069035. In the transformer as shown in FIG. 1, coils are wound around a columnar winding core made of an electrical insulating material. After that, in order to reliably fix the coils, so-called taping is performed, that is, the coils are fixed to the columnar winding core entirely or partly by using adhesive tapes or the like.
In a transformer for a low-voltage, large-current power supply, it is significant to decrease the resistances of the coils, and accordingly the number of turns of each coil must be decreased. If the number of turns is one, since it is very difficult to maintain this one turn by only the coil itself, taping described above becomes important. Taping, however, takes up the winding space of the columnar winding core, and accordingly a columnar winding core having a large winding width is required, resulting in an increase in size of the transformer. In addition, the complicated process of taping increases the cost of the transformer.
Pin terminals to which the two ends of each coil are to be connected are generally arranged in the vicinities of the two end faces of the columnar winding core. Therefore, after the coil is wound, its ends are extracted in directions largely different from the winding direction. For example, when a plurality of coils are to be wound around the columnar winding core, the extracting portions of one coil may come in contact with other coils to further take up the winding space. Thus, the coil winding operation becomes complicated. In particular, for example, when a transformer for a low-voltage, large-current application is to use an electric wire with a large wire diameter, the electric wire is rigid and is difficult to wind, making the operation much more difficult.
When a plurality of electric wires are to be wound around a columnar winding core parallel to each other, a coil which is to be wound at a position farther from the corresponding pin terminals has longer extracting portions. In particular, when the coil has a small number of turns, e.g., one turn, the proportion of the lengths of the extracting portions in the entire length of each coil generally becomes large, and the differences in entire lengths among the respective coils become obvious. Therefore, even when the number of parallel turns of each coil is increased, the resistance of the coil is not decreased so much for the number of parallel turns, and the values of currents flowing through the respective coils differ.
In order to solve these problems, the present applicant has proposed a circuit board with an inductor or transformer (to be sometimes merely referred to as a “transformer” hereinafter) having an arrangement as shown in FIG. 2. FIG. 2 is a plan view of a circuit board with a transformer. One set of coils 6 and 7 are wound around a columnar winding core parallel to each other in one direction, and their ends are connected to lands (output terminals) on the circuit board. The ends of a secondary coil 8 are connected to the pin terminals of a bobbin. With this arrangement, the entire lengths of one set of primary coils can be set almost equal to each other. In the following description, reference numerals Tx (x is a figure) in the drawings represent terminal numbers of a transformer or the like.
In further research of the present inventor, of the ends of the respective coils, those which have different polarities must be arranged close to each other in order to decrease the resistance of a transformer having one set of coils wound around a columnar winding core or the resistance of its peripheral circuit portion. FIG. 3 is a view for explaining the reason for this.
FIG. 3 is a view showing a push-pull switching circuit. A transformer 9 having the arrangement shown in FIG. 2 includes a primary coil (formed of one set of series-connected coils 6 and 7) and the secondary coil 8. The connection point where the opposite-polarity ends of the coils 6 and 7 are connected forms the center tap (CT) of the primary coil. A terminal T1 of the transformer 9, to which one end (non-CT side) of the coil 6 is connected, is connected to the drain electrode of a switching element SW1. Similarly, a terminal T4 of the transformer 9, to which one end (non-CT side) of the coil 7 is connected, is connected to the drain electrode of a switching element SW2. Terminals T2 and T3 of the transformer 9 which form the CT are connected to the positive side of a DC power supply E. The two source terminals of the switching elements SW1 and SW2 are connected to the negative side of the DC power supply E. Thus, a push-pull switching circuit is formed.
An output from the coil 8 as the secondary coil of the transformer 9 is full-wave rectified by, e.g., a diode bridge D1–D4, and is output to the output terminal of the circuit shown in FIG. 3. Generally, capacitors C1 and C2 are connected to the input/output terminals of the circuit shown in FIG. 3, when necessary, in order to suppress fluctuations in the DC voltage. Although a MOSFET is used as a switching element in FIG. 3, the type of the switching element is not particularly limited.
In this push-pull switching circuit, the source electrodes of the switching elements SW1 and SW2 and the terminals T2 and T3 of the transformer 9 which form the CT are desirably arranged close to each other, so that the resistances of the wiring lines among the respective elements may be decreased (FIG. 2 shows general arrangement of the switching elements SW1 and SW2). This demand is strong particularly in a push-pull switching circuit for a low-voltage, large-current application.
Therefore, a decrease in resistance of an electric component and of its peripheral circuit is sought for.